1. Technical Field
The present invention relates to a time synchronization system.
2. Related Art
A network synchronization which synchronizes a time signal (a clock signal) in each of a plurality of stations has been known.
For example, in the related art, as a “Synchronous-Ethernet (a trade mark)”, an optical synchronous network has a hierarchical organization structure in which an “atomic oscillator using a cesium atom”, an “atomic oscillator using a rubidium atom”, a “crystal oscillator”, and the like are disposed in order from a host station to a subordinate station, and is based on frequency synchronization. However, in near future, for the reason of system integration with a mobile base station, it is required to perform highly precise time (phase) synchronization between the stations.
JP-A-2013-207526 discloses a system which includes a GPS receiver installed in each of nodes (stations), detects abnormality of an oscillator, and switches clock paths. By utilizing this system, there is an advantage that it is possible to maintain the frequency of the time signal for each node and phase (time) with high accuracy.
However, the time synchronization system using satellite signals, which is disclosed in JP-A-2013-207526 is required to have a structure which compensates for hold-over when the satellite signal cannot be received, but since a GPS receiver is placed at the same position as that of the structure which compensates for hold-over in the system disclosed in JP-A-2013-207526, the environment in which the satellite signal is received is restricted. In addition, when the GPS receiver and the structure which compensates for hold-over are distant from each other via a wiring or the like, in accordance with a state of a radio wave including the satellite signal or the like, 1PPS, that is, a phase of the time signal fluctuates, and thereby a short-term time (phase) accuracy is deteriorated. Examples of factors that greatly affect the state of the radio wave which is received by the GPS receiver include the influence of the ionosphere, the influence of other radio waves, disposition of a GPS satellite, and an obstacle between the GPS satellite and a GPS antenna.